1. Field of the Invention
The present invention relates to a micellar drive method for recovering petroleum or crude oil (hereinafter referred to as "oil") from subterranean reservoirs. More specifically, it relates to a process for recovering oil from an oil-bearing subterranean reservoir at a high oil recovery efficiency by injecting an aqueous surfactant solution into an oil-bearing subterranean reservoir through an injection well. The aqueous surfactant solution forms a micro-emulsion having a low interfacial tension and drives the oil-bearing subterranean.
2. Description of the Prior Art
A micellar drive method is known in the art as one of the so-called "enhanced oil recovery (EOR)" methods for recovering oil from old or used oil-bearing subterranean reservoirs. According to the micellar drive method, a micellar slug, that is, a clear micro-emulsion derived from water and oil such as petroleum, petroleum distillates, or fuel oil, is injected under pressure into the subterranean reservoirs for the recovery of oil remaining in the subterranean reservoirs. The micellar drive methods are disclosed in, for example, U.S. Pat. Nos. 3,506,070, 3,613,786, 3,740,343, 3,983,940, 3,990,515, 4,017,405, and 4,059,154. These prior arts disclose that various kinds of surfactants including anionic-, nonionic-, and cationic-type surfactants are used as an essential constituent alone or in any mixture thereof in the formation of micellar slugs. Examples of such surfactants are petroleum sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates, alkane sulfonates, polyoxyethylene alkylether sulfates, alpha-olefin sulfonates, polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers, polyol fatty acid esters, alkyltrimethyl ammonium salts, and dialkyldimethyl ammonium salts.
It has been proposed, as an improved micellar drive method in which a micro-emulsion is injected into an oil-bearing subterranean reservoir, that an aqueous solution containing a surfactant and a cosurfactant is injected into a subterranean reservoir to form a microemulsion and to sweep the subterranean reservoir. The aqueous surfactant solution injected into the subterranean reservoir should have the following requirements so as to form a micro-emulsion in the subterranean reservoirs. That is, in order to recover oil from the subterranean reservoir, the aqueous surfactant solution should have: sufficiently low interfacial tensions between an oil and a brine: a viscosity higher, but not too much higher, than those of both subterranean water (or formation water) and oil; and a capability to rapidly form a micro-emulsion upon contact with oil. Furthermore, a surfactant should have a good salinity tolerance and hard-water resistance, since the salt concentration range of the formation water extends widely from a low to high concentration depending upon oil fields. These problems are not satisfactorily solved in the prior art.